This invention relates to the detection or sensing devices used with bubble memory devices. In order to amplify the output of such devices and discriminate between the binary "ones" and "zeros" in the output data stream.
Magnetic bubbles in most magnetic bubble memory systems are detected by a magnetoresistive element placed in the magnetic bubble path on the actual bubble chip. This element changes conductivity slightly in the presence of a bubble in the output from the storage area. This change in conductivity is sensed by passing a continuous current through the detector element and amplifying the resulting voltage change from the change in conductivity. However, the rotating magnetic field that propagates the bubbles on the chip induces a significant voltage in this detector element which may be orders of magnitude greater than the detected bubble signal. To reduce this induction effect, a dummy detector is placed on the bubble chip where it is subjected to the same rotating magnetic field. The two detector elements produce two signals which are supplied to a differential amplifier in such a way that the rotating magnetic field signal common to both the active and dummy detector is cancelled, leaving only signals resulting from actual magnetic bubbles. The dummy detector never receives an input bubble, whereas the active detector element receives the stream of bubbles from the storage area of the memory element.
In addition to the presence of any possible residual output resulting from the rotating magnetic field, there are numerous other sources of noise present in the conventional bubble memory systems which make it comparatively difficult to accurately detect the passing of a magnetic bubble in the device. This invention proposes a detector-amplifier which meets the various problems encountered and produces a reliable strong output signal which accurately reflects the passing of magnetic bubbles in the memory device.
U.S. Pat. No. 3,838,406 shows one type of active-dummy detector used with bubble memory devices. However, the output from such a detector is shown as being processed by only a conventional differential amplifier.
U.S. Pat. No. 3,813,660 shows the use of a strobe pulse used in connection with discriminating between the "ones" and "zeros" in the output of a bubble memory chip by causing activation of the output at only the appropriate time when a relevant signal would be present. However, this patent shows that system used in connection with a radio frequency excited detection system considerably different from the present detection system which is based on direct current sensing of the bubble chip output.